Summary Recent studies have expanded the concept that inflammation is a critical component of tumor progression. It is now clear that the tumor microenvironment largely orchestrated by inflammatory cells, is an indispensable participant in neoplastic process, fostering proliferation, survival and migration. In the last few decades, immunotherapy has become increasingly important in treating cancer. Therefore, there is an urgent need to better understand cancer-immune interactions, particular under specific contexts of cells, tissues and deficient molecular pathways involved. Human hepatocellular carcinoma (HCC), a primary malignancy of the liver and the second-leading cause of cancer mortality worldwide is an example of inflammation-induced cancer. Chronic viral hepatitis, metabolic liver diseases, and alcohol abuse cause chronic inflammation, which induces fibrosis, cirrhosis, and cancer. Macrophages function in the initiation and maintenance of inflammation and fibrosis and tumor associated macrophages (TAMs) play critical roles during cancer progression. However, the cellular and molecular mechanisms underlying reciprocal interaction between macrophages and pre-tumor/ tumor cells remain largely unknown. The Hippo signaling pathway has recently emerged as a major oncosuppressive pathway and play critical roles inhibiting hepatocyte proliferation, survival and HCC formation. Central to the Hippo pathway is the inhibition of Yap/Taz transcription factors by a kinase cascade starting from the Hippo kinase, which are Mst1 and Mst2 in mammals. As macrophage infiltration is dramatically increased in livers with Mst1 and Mst2 removed in hepatocytes, the goal of this proposal is to determine a previously unknown functional mechanism by which Hippo signaling in hepatocytes attenuates hepatocarcinogenesis by inhibiting macrophage infiltration and TAM differentiation. Our preliminary studies have led to the identification of two secreted effectors of Hippo signaling in hepatocytes, monocyte chemoattractant protein 1(Mcp1 or Ccl2) and Jagged 1 (Jag1), that each partially mediates Hippo effects in restricting inflammatory response and tumor growth. We hypothesize that a previously unknown function of Hippo signaling in hepatocytes is to regulate pro-tumor immune response by at least partially inhibiting Mcp1 and Jag1 expression. In Specific Aim 1, we will determine the molecular mechanism underlying TAM differentiation regulated by Hippo signaling in hepatocytes. In Specific Aim 2, we will determine the functions of macrophages in tumorigenesis in the hepatocyte specific Mst1/2 DKO, Mst1/2/Mcp1 TKO and Mst1/2/Jag1 TKO liver. In Specific Aim 3, we will determine the mechanisms whereby Hippo signaling in hepatocytes inhibits expression of Mcp1 and other factors. The knowledge gained from the proposed studies will establish a solid new foundation for further mechanistic investigation of hepatic Hippo signaling in inducing inflammation, tumor microenvironment remodeling and provide new targets and strategies to treat HCC.